Photosynthesizing marine microorganisms can constitute a source of CO2 rather than a sink

Abstract

The unexpected finding that certain major marine photosynthetic microorganisms can serve as a source of CO2 rather than a sink emerged during measurements of inorganic carbon fluxes associated with the CO2-concentrating mechanism. During steady-state photosynthesis, CO2 was evolved at sustained rates up to 5-fold that of photosynthesis; the steady-state external CO2 concentration reached was significantly higher than that at CO2-HCO3- equilibrium. The evolved CO2 originated from HCO3- taken up and intracellularly converted to CO2 in a light-dependent process. Our results bear implications for carbon cycling in the marine environment; the use of naturally-observed stable carbon isotope fractionations as paleobarometer and productivity probe; and for intracellular energy balance and pH regulation.Key words: carbonic anhydrase, CO2 evolution, cyanobacteria, photosynthesis, Synechococcus.